In a conventional semi-additive method of manufacturing a circuit board which includes minute conductive layer, a first thin conductive layer is formed on an insulating substrate, and then a photosensitive resin shaped in a predetermined pattern is provided on the conductive layer. A second conductive layer is formed on a portion of the first conductive layer exposing from the photosensitive resin. After the photosensitive resin on the first conductive layer is removed to allow a portion of the first conductive layer to expose, and the exposing portion is etched off, thus providing the circuit board.
In a method disclosed in Japanese Patent Laid-Open Publication No. 2004-95983, a first metal film is formed on the insulating substrate, and then the metal film is thinned by a chemical polishing treatment to provide a thin base layer. A mask pattern is provided on the base layer, and a portion of the base layer exposing from the mask pattern is electrolytically plated to provide a second metal film. A third metal film is formed on the second metal film by plating. The mask pattern is removed to allow a unnecessary portion of the base layer to expose, and the unnecessary portion is removed, thus providing a circuit pattern including the base layer, the second metal film, and the third metal film. While the removing of the unnecessary portion of the base layer, the second metal film protects a portion of the base layer between the substrate and the second metal film, and prevents the base layer from being removed from the substrate.
In the circuit boards including fine conductor patterns manufactured by the above methods, however, the conductor patterns may have large impedances for high-frequency signals.